Many foods and other products are packaged in pre-portioned amounts in sachets, boxes, cups or bottles or are dispensed onto plates or mixed with other products in different proportions. Portioning into constant amounts prior to dosing into containers is not too much of a problem with powders and fine particulates or with liquids and pastes. More of a problem exists in the packaging of larger particulates such as peanuts, sweets, dried fruits, dried soups, dried cake decorations, pasta pieces, frozen peas, tablets, seeds, parts for engineering, electronic components, office requisites, pelleted petfood and horticultural products.
Portioning such materials into constant amounts for dosing into containers tends to be expensive and time consuming using available technology which in general is expensive and less than ideal for the task.
Some regular free flowing particulates can be portioned reasonably accurately by volume. Methods range from a simple measuring cup to automated conveyor belts consisting of a succession of cups which are filled and swept level. Mechanical volumetric slide gate dispensers are often used and in another method a quasi-constant flow of material is arranged to fall from a vibrator tray onto a "star wheel" which collects approximately equal amounts in each segmented compartment for subsequent release. Where such volumetric dispensing methods are satisfactory, they are probably the best choice since they are cheap and easy to maintain. However, for many applications, volumetric methods are not accurate enough since the materials do not pack equally into spaces, and examples of such products are wrapped sweets, dried vegetables and sea foods such as prawns. For these a weighing method has to be used in order to comply with current consumer legislation and not incur losses due to overpack and two weighing methods are in general use, linear weighing and combinatory weighing.
In linear weighing, a weigh bucket is mounted on an electronic weighing machine and a feeder such as a vibrator or conveyor belt is used to dispense material into the weigh bucket. As material accumulates in the bucket an increasing weight signal is received. The feeder activity such as vibrator amplitude is controlled until a given point is reached determined by the feedback weight signal at which stage it changes to a dribble feed until the right amount is reached at which point the feed is cut off. The product is then released from the bucket which is then rendered available to receive the next dose. The process is time consuming and if high dosing rates are required banks of linear weighers must be employed which render the whole exercise large and expensive and complex.
Where faster weighing is required with larger particulates and where an accuracy of better than a single particle is required combinatory weighing techniques must be employed. Here a computer is employed to select from a plurality of weigh buckets each containing relatively small quantities of the product involved, so as to produce a combination of weights which is nearest to the required weight. The chosen buckets are then discharged and refilled for a subsequent cycle.
Belt weighing techniques are also employed using a load cell and totaliser.
A further weighing apparatus involves changing the direction of the flow of bulk material by causing material to fall onto an inclined plate which is connected to a load cell and integrating circuit to produce an indication of mass flow.
A mass flow measuring system is also known in which the material is caused to react with a rotating disc, thereby imparting an inertial load on the disc drive system which can be measured as an increased torque demand. This demand can be measured and interpreted as mass flow rate. Such a system is described in U.S. Pat. No. 2,771,773, for example. However, the known system is particularly related to heavy industry and is suitable for the metering of such materials as coal, iron ore, bulk chemicals, etc. No equipment of the kind described in the U.S. patent and suited to light industry and packaging is known and no equipment for application in these fields is currently available. This is doubtless due to the fact that measurement of the forces generated by the passage of a few grams of material calls for an entirely different approach to that required for the measurement of tons of material. It is to the specific area of light weights of material that the present application is directed.
It is an object of the present invention to provide a simpler and faster mass flow measuring system employing a rotating disc, capable of being made sufficiently sensitive to enable its application to light weights of material.